Planar lightwave wavelength device using moveable mirrors

ABSTRACT

A method and apparatus are disclosed for adjusting the phase of an optical signal by varying the path length of the optical signal using one or more moveable mirrors. The phase adjustment techniques of the present invention may be employed in various optical devices, including 1×n optical switches. The position of the mirrors may be controlled, for example, using micromachined control elements that physically move the mirror along the lightpath. An exemplary 2-by-2 optical switch includes two waveguides configured to include a coupler region. A mirror is positioned at the output of each waveguide. The position of at least one of the mirrors may be adjusted along the optical path and the mirrors reflect the light exiting from the end of the waveguides back into the same waveguide after an adjustable phase delay due to the round trip through an adjustable air gap between the waveguides and corresponding mirrors. A received optical signal is split in the coupler region into two generally equal components and the phase of at least one component of the optical signal is adjusted by controlling the relative position of the mirrors. The optical components are then recombined and the optical signal appears at the appropriate output port of the optical switch. The present invention may also be applied in wavelength selective optical switches that support multiple optical channels. A number of techniques are also disclosed for fabricating optical devices in accordance with the present invention.

FIELD OF THE INVENTION

[0001] The present invention relates to mechanisms for manipulatinglight in optical circuits and in waveguide chips and, more particularly,to optical devices for routing multi-wavelength optical signals.

BACKGROUND OF THE INVENTION

[0002] Many innovations for optical communication systems have involvedthe manner in which light waves are switched and manipulated. In manyoptical transmission applications, it is necessary to perform one ormore of the following actions on light: switching, attenuation, routingto different locations or manipulating the phase of light. Such actionsare critical for realization of the optical networks that are thefoundation of global communications systems.

[0003] Optical communication systems increasingly employ wavelengthdivision multiplexing (WDM) techniques to transmit multiple informationsignals on the same fiber, and differentiate each user sub-channel bymodulating a unique wavelength of light. WDM techniques are being usedto meet the increasing demands for improved speed and bandwidth inoptical transmission applications. In optical communication networks,such as those employing WDM techniques, individual optical signals areoften selectively routed to different destinations. Thus, a highcapacity matrix or cross-connect switch is often employed to selectivelyroute signals through interconnected nodes in a communication network.

[0004] At the heart of these cross-connect switches is the singleswitching unit. Single switching units should exhibit low manufacturingand operation costs, small losses of the optical signal when passingthrough the switch (low insertion loss), and high blocking of unwantedsignals (high extinction ratio). Many switches used in opticalcommunication networks are manual, and are relatively cheap tomanufacture, but expensive to operate. In addition, available switchestend to prevent high switching speed and flexibility. Electronicswitches first convert the optical signal into an electronic signal,perform the switching and then convert back into optical signals. Theseconversions are very expensive and the switches are complex to managebut allow considerable flexibility. As networks grow and become dense,however, electronic switches become increasingly expensive and harder tofabricate.

[0005] Therefore, optical switches that operate directly on the lightwave are favorable. Optical switches are often realized in opticalwaveguides that can be manufactured with low cost and enable easymultiplexing and de-multiplexing of the WDM signal using waveguidegrating routers (WGR). For a detailed discussion of waveguide gratingrouters, such as optical star couplers, see U.S. Pat. No. 4,904,042 toDragone. Switching in waveguides is often accomplished by applying phaseor amplitude changes using an electrooptic effect or a thermoopticeffect. The electrooptic effect usually requires special and expensivewaveguide materials, such as InP or LiNbO₃, that exhibit nonlineareffects and are used for fast switching and specialized applications.Thermooptic switching (a heat induced change in the index of refraction)in waveguides is robust and is extensively used in combination with WGRin optical waveguide circuits. However, thermooptic switches suffer fromhigh power consumption and limit the complexity of circuits that can bebuilt due to thermal crosstalk and maximum power limitations.

[0006] Recently, micro electro mechanical systems (MEMS) switches havebeen introduced for network applications. MEMS switches are usuallymovable mirrors that change the propagation direction of light, or blocklight. For a discussion of a wavelength-selective add-drop multiplexerthat uses movable mirrors to add and/or drop spectral components from awavelength-division-multiplexed optical signal, see, for example, U.S.Pat. No. 5,974,207 to Aksyuk et al, assigned to the assignee of thepresent invention and incorporated by reference herein. To change thepropagation direction of the light, or block the light, a shutter mustbe moved a distance long enough to move the shutter in and out of alight beam or tilt the shutter with an angle larger than the angularwidth of the optical beam. These displacements are usually challengingto make with MEMS actuators that excel at microscopic motion. Ifswitching can be achieved by motion that is the size of the opticalwavelength (about 1-2 μm for common communications systems), MEMSswitches could be implemented in waveguides and other systems.

SUMMARY OF THE INVENTION

[0007] Generally, a method and apparatus are disclosed for adjusting thephase of an optical signal by varying the path length of the opticalsignal using one or more moveable mirrors. The phase adjustmenttechniques of the present invention may be employed in various opticaldevices, including 1×n optical switches that introduce a phase changeand recombine the optical signal to switch a received optical signal toa desired output port. This phase changing method can also be employedfor pulse-shaping applications, where phase changes of the differentspectral components of a wave are phase delayed in different amounts, aswell as for dispersion compensation devices, polarization manipulationdevices, and other apparatuses where a phase change is required.

[0008] In an exemplary 2-by-2 optical switch, two waveguides configuredto include a coupler region carry light signals in both directions. Amirror is positioned at the output of each waveguide. The position of atleast one of the mirrors may be adjusted along the optical path and themirrors reflect the light exiting from the end of the waveguides backinto the same waveguide after an adjustable phase delay due to the roundtrip optical path through an adjustable air gap between the waveguidesand corresponding mirrors. A received optical signal is split in thecoupler region into two generally equal components. Thereafter, thephase of at least one component of the optical signal is adjusted inaccordance with the present invention, by controlling the relativeposition of the mirrors to introduce a phase change. The optical signalcomponents are then recombined in the coupler region to accomplishconstructive or destructive interference, based on the introduced phasechange. In this manner, the optical signal appears at the desired outputport.

[0009] The position of the mirrors may be controlled, for example, usingmicromachined control elements that physically move the mirror along thelight path. The present invention may also be applied in wavelengthselective optical switches that support multiple optical channels. Anumber of techniques are disclosed for fabricating optical devices inaccordance with the present invention.

[0010] The present invention thus combines phase-senstivite waveguidestructures with micromachined actuators that move small amounts torealize a switch. The present invention can be used for any lightswitching application, in addition to the exemplary communicationsapplications, as would be apparent to a person of ordinary skill in theart. For example, the present invention can be applied to make anadd-drop multiplexer (ADM). An ADM is often needed in an optical networkwhen it is desirable to remove (drop) light of a given wavelength from afiber or add light of a given wavelength to the fiber.

[0011] A more complete understanding of the present invention, as wellas further features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 illustrates an exemplary conventional (prior art) 2-by-2Mach-Zhender interferometer optical switch;

[0013]FIG. 2 illustrates an exemplary 2-by-2 optical switch inaccordance with the present invention;

[0014]FIG. 3 illustrates an optical switch that includes the opticalswitch of FIG. 2 and at least one optical circulator to separateincoming and outgoing light;

[0015]FIG. 4 illustrates a block diagram of 2-by-2 wavelength selectiveoptical switch in accordance with the present invention that supports noptical channels;

[0016]FIG. 5 illustrates an exemplary implementation of the wavelengthselective switch of FIG. 4;

[0017]FIG. 6 is a schematic diagram of a micro electromechanical systems(MEMS) mirror that may be fabricated using silicon technologies inaccordance with one embodiment of the present invention;

[0018]FIG. 7A is a side view of an embodiment of a monolithic opticalswitch in accordance with the present invention;

[0019]FIG. 7B is a top view of the embodiment of a monolithic opticalswitch fabricated in accordance with the present invention showing someof the design parameters; and

[0020]FIGS. 8A through 8H, collectively, illustrate side and top viewsof an exemplary process for fabricating the optical switch of FIGS. 7Aand 7B.

DETAILED DESCRIPTION

[0021]FIG. 1 illustrates an exemplary conventional 2-by-2 Mach-Zhenderinterferometer optical switch 100 having two input ports 110-1, 110-2and two output ports 120-1, 120-2, two 3 dB couplers 151 and 152, and atleast one thermooptic phase shifter 140. Generally, the optical switch100 accepts an incoming signal at an input port 110-1 or 110-2 andselectively passes the optical signal to one of the output ports 120-1or 120-2. (For a discussion of Mach Zhender interferometers, see, forexample, Katsunari Okamoto, “Fundamentals of Optical Waveguides,” p.159, Academic Press (2000)).

[0022] Generally, the optical switch 100 accepts an incoming signal ofmultiple wavelength channels at an input port 110-1 or 110-2, which isthen split into two equal parts in waveguides 130-1 and 130-2 at the 3dB coupler 151. The phase of the signal in waveguide 130-1 can bechanged, affecting the way in which the signals interfere whenrecombined at the second coupler 152 to selectively pass the opticalsignal to one of the output ports 120-1 or 120-2 or divide the intensitybetween them. Typically, the phase change is achieved by the thermoopticeffect with heater 140 by varying the temperature of the waveguide 130-1in which the optical signal travels. It has been found, however, thatthe necessary temperature change requires significant power consumptionand significant cross-talk between nearby switches on the same chiplimiting the amount of switches that can be put on one chip and thecomplexity of a switch system that can built.

[0023] According to one feature of the present invention, a phase changeis achieved in an optical signal by varying the optical path length ofthe signal using one or more moveable mirrors. FIG. 2 illustrates anexemplary 2-by-2 optical switch 200 in accordance with the presentinvention. As shown in FIG. 2, the optical switch 200 has two waveguides210 and 220, each carrying light in both directions. The two waveguides210 and 220 are configured to include a coupler region 225, in a knownmanner. As discussed hereinafter, the optical switch 200 is configuredin a reflective mode (this also helps in reducing by a factor of 2, thenecessary chip area needed for the switch). Thus, an input to a singlewaveguide, such as the input 210-i (or 220-i) to the waveguide 210 (or220), is both an input port and an output port of the optical switch200.

[0024] As shown in FIG. 2, mirrors 230, 240 are positioned at the outputof each waveguide 210, and 220. The position of at least one of themirrors 230, 240 may be adjusted along the optical path. The mirrors230, 240 reflect the light exiting from the end of the waveguides backinto the waveguides 210, 220 after an adjustable phase delay due to theround trip through the adjustable air gap 250 between the waveguides210, 220 and mirrors 230, 240, respectively. It is noted that the gap250 can also be filled with index matching material to get moreefficient coupling in and out of the waveguides. However, diffractionlosses can be minimized by reducing the gap 250 to a necessary minimum.

[0025] Generally, an optical signal applied to the input of a singlewaveguide, such as the input 210-i to the waveguide 210, is split in thecoupler region 225 into two generally equal components. Thereafter, inaccordance with the present invention, the phase of at least onecomponent of the optical signal is adjusted, as desired, by controllingthe relative position of the mirrors 230, 240 to introduce a relativephase change in the reflected light. The optical components are thenrecombined in the coupler region 225 to accomplish constructive ordestructive interference, based on the introduced phase change. In thismanner, the optical signal appears at the appropriate output port of theoptical switch 200.

[0026] As previously indicated, each waveguide 210 and 220 in theoptical switch 200 of FIG. 2 potentially carries light in bothdirections. FIG. 3 illustrates an optical switch 300 that includes theoptical switch 200 of FIG. 2 and at least one optical circulator 310that separates incoming and outgoing light, in a known manner. Theexemplary optical switch 300 includes one optical circulator 310connected to the bottom waveguide 220 of the optical switch 200. In thismanner, the optical circulator 310 allows bi-directional communicationon the waveguide 220.

[0027]FIG. 4 illustrates a 2-by-2 wavelength selective switch 400 thatsupports n optical channels. As shown in FIG. 4, the optical switch 400includes two bi-directional waveguides 410, 420, a multiplexing phaseswitch 450 and a mirror array 480 having p mirrors. Input light fromwaveguides 410 or 420 is separated into the different intermediate parts460-o-l to 460-o-p (usually in waveguides) by the multiplexing phaseswitch 450, this light impinges on the mirror array 480 and is reflectedback into the multiplexing phase switch 450 that channels the signal tothe waveguides 410 or 420. The phase of one or more of the n opticalchannels is adjusted by varying the position of one or more mirrors inthe mirror array 480. In this manner, each of the different componentwavelengths can be selectively switched to either waveguide 410, 420.One embodiment of the multiplexing phase switch 450 is discussed belowin conjunction with FIG. 5.

[0028]FIG. 5 illustrates an exemplary implementation of the multiplexingphase switch 450 of FIG. 4. As previously indicated, the presentinvention achieves switching using the destructive interference of twooptical components of the same optical signal. However, in order to getgood extinction ratio the intensity splitting has to be very accurateusually barred by manufacturing tolerances. Thus, it is known to employthree or more copies (orders) of the same optical signal to cancel theoptical signal, removing the limitations due to manufacturingdifficulties. As shown in FIG. 5, the multiplexing phase switch 450includes a first star coupler 510, an array of waveguides 540 and asecond star coupler 550.

[0029] The first star coupler 510 splits the incoming signals intodifferent similar parts. The waveguide array 540 includes n waveguideswith different lengths to enable the multiplexing at the second starcoupler. A second star coupler 550 focuses each of the n channels andcreates m copies (orders) of each channel. Thus, at the output of thesecond star coupler 550, there are m copies of each of the n channels.Thus, the waveguide array 570 includes m×n independent waveguides andthe mirror array 580 has m×n mirrors. In one implementation, there are 3copies of each of the n channels (m=3) to provide +1^(st) order, 0 orderand −1^(st) order copies of each channel. In one exemplaryimplementation, the 0 order copy of the optical signal includes 40% ofthe optical intensity while the +1^(st) order and −1^(st) order copiesof each channel have 30% of the optical intensity (this ratio variesbetween channels). The phase of each of the m copies of the n channelsis adjusted independently in accordance with the present invention byvarying the position of the corresponding mirror in the mirror array580. This is fully described in U.S. Pat. No. 6,049,640 to Doerr,entitled “Wavelength-Division-Multiplexing Cross-Connect Using AngularDispersive Elements and Phase Shifters,” incorporated by referenceherein.

[0030]FIG. 6 is a schematic diagram of a micro electromechanical systems(MEMS) mirror assembly 600 that may be fabricated using silicontechnologies in accordance with one embodiment of the present invention.As shown in FIG. 6, the mirror assembly 600 includes a reflectiveportion 610 that is held in position using four silicon springs 615-a,b, c, d, and is kept at ground potential. A voltage V is applied to anelectrode 620 underneath the mirror to move the mirror closer to theelectrode. Fabrication is done, for example, using a three layerpolysilicon surface micromachining process similar to the one discussedin D. Keoster et al., “Multiuser MEMS Processes (MUMPS) Introduction andDesign Rules,” Rev. 4, MCNC MEMS Technology Applications Center,Research Triangle Park, N.C. 27709 (Jul. 15, 1996), incorporated byreference herein.

[0031] An electrically-controlled movable mirror capable ofaccomplishing similar function may have different design and may befabricated by a variety of different micromachining techniques. Forexample, a suitably reflective suspended movableelectrostatically-controlled membrane can be used instead of areflective plate suspended on microfabricated springs, as described inU.S. Pat. No. 5,949,571, entitled “Mars Optical Modulators, incorporatedby reference herein.

[0032] As described in the previous paragraph, the moving mirrormanufacturing is by a process separate from the waveguide manufacturingprocess. This enables the flexibility to optimize both processes for thewaveguide manufacturing and mirror array manufacturing and use existingwell-proven manufacturing processes at the expense of having tointegrate the two chips later. This is done by active alignment of thetwo pieces and attachment by an adhesive, solder or other similartechnique. However, as discussed below in conjunction with FIGS. 7A and7B, an embodiment of the invention is presented where the MEMS mirrorsare manufactured on the waveguide chip enabling a monolithic switch.

[0033]FIG. 7A is a side view of an embodiment of a monolithic opticalswitch 700 in accordance with the present invention. As shown in FIG.7A, the optical switch 700 is fabricated in the waveguide layer 710 on asubstrate 720. The waveguide layer 710 is comprised on an upper andlower cladding 712, 714 and a higher index core glass 718 within whichthe light is guided. An exemplary process for fabricating the opticalswitch 700 is discussed further below in conjunction with FIG. 8.

[0034] The optical switch 700 includes a mirror 750 that may be embodiedas a reflective material, such as gold, deposited on the cladding andcore material. The position of the mirror 750 is varied by applying avoltage to the terminal, V, as shown in FIG. 7A. The mirror 750 is shownin FIG. 7A in a default position, with no voltage applied. As theapplied voltage increases towards a maximum value, the mirror 750 movesto the right in the figure, towards the grounded electrode 760. As shownin FIG. 7A, the mirror 750 is positioned at the output of waveguide core718 and may be adjusted along the optical path. The mirror 750 reflectsthe light exiting from the end of the waveguide 718 back into thewaveguide 718 after an adjustable phase delay due to the round tripthrough the adjustable air gap 740 between the waveguide 718 and mirror750. It is noted that the optical path of the light may expand in theair gap 740 therefore a minimal gap 740 is desired. Index matching fluidor beam shaping at the end of the waveguide can be used to relax thisconstraint.

[0035]FIGS. 8A through 8H illustrate an exemplary process forfabricating the optical switch 700 of FIGS. 7A and B. As shown in FIGS.8A (side view) and 8B (top view), the process is initiated withwaveguides 810 having a lower and upper cladding and a higher index coreglass, deposited on a substrate 820. Thereafter, as shown in FIGS. 8C(side view) and 8D (top view), metal is deposited on the waveguides 810for the electrical connections (ground (0V) and V) discussed above inconjunction with FIGS. 7A and 7B.

[0036] As shown in FIGS. 8E (side view) and 8F (top view), two holes arethen etched in the glass 810 to form the front and rear surfaces of themirror 750. In addition, the substrate 820 is released, for example, bywet etching through the holes that were etched to remove portions of thesubstrate, to avoid a short on the bottom and allow movement of themirror 750. Finally, as shown in FIGS. 8G (side view) and 8H (top view),angular depositions are applied to the etched mirror to provide areflective and electrode surfaces using shadow mask evaporation.

[0037]FIG. 7B illustrates a top view of an exemplary embodiment of anoptical switch 700 fabricated in accordance with the present invention.It is generally desired to be able to move the mirror a distance of$\frac{\lambda}{2}$

[0038] (for a round trip phase shift of 2π). Thus, for typicalwavelengths of 1.5 μm, it is generally desired to be able to move themirror a distance of larger than 0.75 μm. Mirror movement of 1.9 μm isobtained in an exemplary implementation where the membrane length, L, is2×10⁻⁴ m, the membrane thickness, t, is 2×10⁻⁶ m, the trench width(electrode spacing), d, is 2×10⁻⁶ m and the maximum applied voltage, V,is 100V. In this embodiment, the maximum distance, Y, that a mirror canbe moved is obtained from the approximate formula as follows:${Y \cong {\frac{1}{64} \cdot \frac{ɛ}{E} \cdot \frac{L^{4} \cdot V^{2}}{t^{3} \cdot d^{2}}} \cong {1.2\quad \mu \quad m}},$

[0039] where ε≅8.85×10⁻¹² F/m is the dielectric constant of air andE≅73GPa is Young's modulus (a property of the silica glass).

[0040] It is to be understood that the embodiments and variations shownand described herein are merely illustrative of the principles of thisinvention and that various modifications may be implemented by thoseskilled in the art without departing from the scope and spirit of theinvention. For example, in one variation, the mirror can be positionedat a variable angle so that the light returned to the waveguide can beattenuated by a desired amount by deflecting a certain portion of thelight so that it is not captured by the waveguide. Also the actuationmechanism of the mirror may be changed to thermal actuation, magneticactuation or other by modification of the mirror actuators 700 or 600accordingly. Another variation can change the nature of the waveguide tobe manufactured from different materials like polymers. Anothervariation may include partially reflecting mirrors enabling Fabry-Perotlike interferometers.

We claim:
 1. An optical device, comprising: at least one waveguide forcarrying an optical signal; and at least one mirror having an adjustableposition to vary a path length of said optical signal.
 2. The opticaldevice according to claim 1, wherein said mirror is controlled by amicromachine control element that positions said mirror in a desiredposition along an optical path.
 3. The optical device according to claim1, wherein said mirror is positioned at an end of said at least onewaveguide.
 4. The optical device according to claim 1, wherein saidmirror is fabricated in the waveguide material deposited on a substrate.5. The optical device according to claim 1, wherein said optical signalis a wavelength-division multiplexed (WDM) signal comprising Nwavelength channels and wherein said optical device further comprises ademultiplexer for producing a plurality of demultiplexed output signalsfrom said input WDM signal and at least one mirror associated with eachof said N wavelength channels.
 6. The optical device according to claim5, wherein a plurality of said waveguides carry each of said Nwavelength channels.
 7. A method for adjusting a phase of an opticalsignal, said method comprising the steps of: receiving said opticalsignal; and adjusting a position of a mirror along a path of saidoptical signal.
 8. The method according to claim 7, wherein saidadjusting step is performed by a micromachine control element thatpositions said mirror in a desired position along an optical path. 9.The method according to claim 7, wherein said mirror is positioned at anend of at least one waveguide.
 10. The method according to claim 7,wherein said mirror is fabricated from a waveguide deposited on asubstrate.
 11. The method according to claim 7, wherein said opticalsignal is a wavelength-division multiplexed (WDM) signal comprising Nwavelength channels and wherein said method further comprises the stepof demultiplexing said optical signal to produce a plurality ofdemultiplexed output signals from said input WDM signal.
 12. An opticalswitch, comprising: means for receiving said optical signal; means forsplitting said optical signal into at least two optical components; amoveable mirror for adjusting a phase of at least one of said opticalcomponents by adjusting a position of said mirror along a path of saidoptical component; and means for recombining said at least two opticalcomponents.
 13. The optical switch of claim 12, wherein said means forreceiving comprises at least one waveguide for carrying said opticalsignal.
 14. The optical switch of claim 12, wherein said means forsplitting and recombining said optical signals is a coupler regionbetween two adjacent waveguides, a star coupler, an arrayed waveguiderouter or a multimode interference waveguide.
 15. The optical switch ofclaim 12, wherein said mirror is controlled by a micromachine controlelement that positions said mirror in a desired position along anoptical path.
 16. The optical device of claim 12, wherein said mirror ispositioned at an end of said at least one waveguide.
 17. The opticaldevice of claim 12, wherein said mirror is fabricated from waveguidematerial deposited on a substrate.
 18. The optical device of claim 12,wherein said optical signal is a wavelength-division multiplexed (WDM)signal comprising N wavelength channels and wherein said optical switchfurther comprises a demultiplexer for producing a plurality ofdemultiplexed output signals from said input WDM signal and at least onemirror associated with each of said N wavelength channels.
 19. A methodfor switching an optical signal, said method comprising the steps of:receiving said optical signal; splitting said optical signal into atleast two optical components; adjusting a phase of at least one of saidoptical components by adjusting a position of a mirror along a path ofsaid optical component; and recombining said at least two opticalcomponents.
 20. The method according to claim 19, wherein said adjustingstep is performed by a micromachine control element that positions saidmirror in a desired position along an optical path.
 21. The methodaccording to claim 19, wherein said optical signal is awavelength-division multiplexed (WDM) signal comprising N wavelengthchannels and wherein said method further comprises the step ofdemultiplexing said optical signal to produce a plurality ofdemultiplexed output signals from said input WDM signal.